EP0097275B1 - Method of operating a hydrostatic or pneumatic drive, as well as a drive - Google Patents

Description

The invention relates to a method for operating a hydrostatic or pneumatic drive for masses to be accelerated and braked, comprising a linear or rotary motor, a hydraulic or pneumatic pressure source and a valve arrangement, the passage opening of which is controllable to deliver a variable working current, the changing working pressures is subject to a compression volume of the hydraulic or pneumatic medium compared to the initial state, depending on the expected working pressure. The invention also relates to a related drive, which is provided with a setpoint control generator for controlling the valve arrangement.

In a known control device for a hydraulic system (DE-A-2 902 264), a defined acceleration or deceleration of the drive can be achieved by a defined increase or decrease in the pressure medium flow. Various electronic control elements ensure that acceleration surges are avoided. In particular, a four-way valve with three switch positions is provided between the pump and the motor, which can be controlled electrically and contains ramp generators in the electrical control arrangement. The output signal of the ramp generator is fed to an electrical switching amplifier that controls the directional valve. The fact that the hydraulic fluid is compressible and the expansion of the drive components when the pressure increases are not taken into account.

In another known positioning device for a hydraulic drive (DE-A-2 808 694), a sudden change in speed is to be avoided. For this reason, a ramp generator is provided between the setpoint control generator and the actual value scanner on the one hand and an electrically controllable hydraulic actuator on the other hand. The compression volume is not taken into account.

In injection molding, it is known (FR-A-2 299 103) that an injection mold should be filled more slowly at the end of the filling stroke, because otherwise a pressure plate would result. Therefore, the speed of the injection plunger of the injection molding machine is reduced before the full filling is reached.

In pressure and injection molding machines, a stationary or moving mass is driven by means of a piston-cylinder unit in order to be brought to a certain speed. For this purpose, for example, a throttle is increasingly turned on in order to apply increasing pressure to the piston that drives the mass. However, the hydraulic fluid is not incompressible, and the components of the drive can expand, in particular the hydraulic lines, so that a certain proportion of the hydraulic flow supplied is capacitively absorbed by the drive system and is not immediately noticeable in a shift in the mass. This capacitively absorbed hydraulic current represents a tensioned spring, which together with the mass results in an oscillatory structure. The faster the desired final speed is reached, the greater the vibration excitation. So if you operate the setpoint control generator with a steep ramp between the different setpoints, the system responds with weakly damped vibrations, i. H. the pressure gradually settles to its setpoint, as does the speed of the piston. In practice, a ramp of the order of 400 ms is therefore observed. Furthermore, the view has been taken that the cycle time of injection molding machines into which the transition areas between different speeds enter cannot be significantly reduced ("Microelectronics in Injection Molding Machines" by Dr. W. Elbe and RK Jackson, company publication by Mannesmann-Demag and Mannesmann-Demag-Hamilton, in particular page 4).

The invention has for its object to improve the dynamics of a hydrostatic or pneumatic drive in order to shorten the machine cycle time and / or to reduce the stresses on the machine. If necessary, drive energy can also be saved because additional, energy-consuming damping measures can be omitted. In the case of pressure and injection molding machines, for example, lower back pressures can be used.

The object is achieved on the basis of the measures of claims 1 and 6, respectively.

• Fig. 1 einen erfindungsgemäßen Antrieb in schematischer Darstellung,
• Fig. 2 ein Diagramm der Steuerspannung U, des Arbeitsstroms 011, der zeitlichen Ableitung D des Arbeitsstroms, der zugehörigen Drucks P, des abgeführten Hydraulikstroms 0.₁₂. des zugehörigen Gegendrucks G, der Verschiebung S, der Geschwindigkeit V und der Beschleunigung B der Masse über der Zeit in ms,
• Fig. ein weiteres Diagramm des Steuerstroms U, des Arbeitsstroms 011, des zugehörigen Drucks P, des abgeführten Arbeitsstroms A, des zugehörigen Gegendrucks G und der Geschwindigkeit V über der Zeit t in ms.

An embodiment of the invention is described with reference to the drawing. It shows

• 1 shows a drive according to the invention in a schematic representation,
• 2 shows a diagram of the control voltage U, the working current 011, the time derivative D of the working current, the associated pressure P, the hydraulic current 0. ₁₂ . the associated back pressure G, the displacement S, the speed V and the acceleration B of the mass over time in ms,
• Fig. Another diagram of the control current U, the working current 011, the associated pressure P, the discharged working current A, the associated back pressure G and the speed V over time t in ms.

The main components of the drive arrangement shown in Fig. 1 are a piston-cylinder unit 1 as a hydrostatic or displacement motor, a pump 2 and an associated controllable pressure relief valve 3 as a hydraulic pressure source, a pressure difference constant valve 4, an electrically controllable valve arrangement 5 and a setpoint control generator 6. An exhaust valve 7 can be connected to the piston-cylinder unit 1. It goes without saying that conventional pressure limiting valves can be connected to protect the system, which, like another pump with additional valves for higher hydraulic power consumption, are not shown.

The pump 2 pumps hydraulic fluid into a delivery line 10 which extends through the valve arrangement 5 and continues as an input line 11 to the piston-cylinder unit 1, the output line of which is designated by 12. The pressure building up in the delivery line 10 is supplied via a control line 13 and a series of throttles to the rear of the valve 3, which is connected to the valve 4 via a further control line 14. A connection to the input line 11 is created via a further control line 15. In this way, the pressure difference is present at the valve 4, which results as a pressure drop at the valve arrangement 5 between the lines 10 and 11. If this pressure drop exceeds a certain value of, for example, 2 bar, valve 4 opens and regulates the control pressure at valve 3. This tries to maintain the set pressure difference of 2 bar, i. H. If the pressure in line 10 increases by more than 2 bar compared to line 11, valve 3 responds and relieves line 10 accordingly. A constant pressure drop is thus generated at the valve arrangement 5.

The valve arrangement 5 represents a so-called proportional throttle, i. H. the width of its throttle opening 20 depends proportionally on the size of the supplied setpoint control voltage U when a certain control time has elapsed. In particular, this can be achieved in that a setpoint-actual value comparator 21 compares the setpoint U supplied by the control generator 6 with the actual value of the throttle opening 20, which is obtained by means of a displacement scanner 22. It is readjusted until the actual value matches the setpoint. In practice, the throttle opening 20 is formed by a main valve, which is controlled via a pilot or pilot valve, which need not be explained in detail.

If the throttle 20 is opened increasingly, which can be done by a ramp-like setpoint signal U, the pressure in the line 11 increases and the linear displacement motor 1 starts to move. However, not only is the cylinder space enlarged in accordance with a flow Q ₁ , but due to the compressibility of the hydraulic fluid and the expansion of the components with increasing pressure, a hydraulic capacity C _{1 is} filled to a certain extent with the flow Q _C1 , which is derived from the working flow Q ₁₁ is fed. The volume taken up is referred to as the compression volume V _k . The capacitance Ci represents an energy store in the sense of a tensioned spring and together with the mass M to be driven results in an oscillatory structure. Although this can be damped by a brake formed by the discharged hydraulic flow Ü12 and the throttling outlet valve 7, a capacity C ₂ must also be assigned to the outlet side, which in turn represents an energy store which can give rise to further vibration excitation.

Surprisingly, it has been found that the vibration excitation is largely avoided if the setpoint control voltage U has the course shown in FIGS. 2 and 3.

The setpoint voltage generator 6 can contain two pulse generators for generating the positive and negative pulses and a ramp generator which provides the reference voltage for the pulse generators, so to speak. Both the pulse width and the pulse height of the pulse generators as well as the ramp slope and ramp height of the ramp generator can be set. In the event of changing requirements, as is the case in pressure and injection molding machines, a microprocessor 8 can be provided in order to control the pulse generators and the ramp generator accordingly. In this way, the physical quantities that determine the compression volume V _k can be taken into account precisely for each application. Furthermore, it can be determined whether additional pulses I _G and I _R are switched on or whether the switching on of a pulse is completely omitted, as is the case with the phase-out or positive locking.

Fig. 2 shows a time-dependent voltage ramp U, a positive control pulse I ₁ and a negative control pulse 1 _{2 are} superimposed at the beginning of the ramp increase and at the end of the ramp increase. The size of this positive and negative control pulse I ₁ and I ₂ depends on the respective compression volume V _k , as will be explained below.

If the piston of the engine 1 has a side A _i exposed to the supplied pressure p ₁ and a side A ₂ facing away from the pressure p ₂ , the following equation can be established for the forces:

If one neglects the frictional force F _R and the counterforce G = p ₂ · A ₂ , the shortened equation of force remains

The pressures p ₁ , p ₂ are not constant over time, which results in capacitive hydraulic flows Q _C1 and Q _C2 that change over time, corresponding to Qc = p · C. The speed equation is therefore:

und eine verkürzte Kraftgleichung

If the pressure supplied changes from an initial value pia to an end value pie, the following compression volume results:

and a shortened equation of force

If one assumes a ramp-like speed s _R during the pressure rise time t _R and thus an initial value Q _1a and an end value Q _{1e of} the hydraulic current Q ₁ , the following results for the acceleration S _R :

The compression volume is calculated from this:

This compression volume V _k is applied to a certain extent at the beginning of the ramp, ie a setpoint voltage pulse li is given, the content of which corresponds to this compression volume V _k . To take the counterforce G and the frictional force F _R into account, additional pulses I _G and I _{R are applied} . The size of these pulses I _G and I _R is determined empirically.

The area of the voltage pulse I ₂ at the end of the rise ramp corresponds to the pulse I ₁ as calculated. If the values M, C ₁ and A are assumed to be machine-given in the application, the pulse area I ₁ or I ₂ depends only on the voltage difference and the ramp steepness of the setpoint voltage U. So if you increase the slope of the ramp to shorten the machine cycle time, you have to superimpose a larger pulse I ₁ or I _{2 on} the ramp. It follows that a small voltage pulse must be superimposed on a ramp part that is flat in time, which can also be omitted in such a case, as shown in FIG. 3, right-hand side of the diagram.

The invention is also applicable with respect to ramping away. The size of the pulse 1 ₃ (Fig. 3) can be determined by analogous considerations, namely to:

K _s denotes the gradient of the ramp function U, while the other symbols have the meaning already explained. For the sake of illustration, however, the D curve is mirrored at the t coordinate, ie the values are actually negative. At the end of the ramp down there should be a positive pulse 1 ₄ . However, since the speed should go towards 0, such a control pulse is not superimposed on the setpoint control voltage. On the other hand, if it is to be brought down to a finite speed, such a positive control pulse 1 _{4 is} used.

2 shows the characteristic values of the drive for starting from a standstill. The mass to be driven only starts to move when a certain pressure p has built up. The acceleration B starts, reaches its highest value at about the mean of the ramp and then drops to 0. The curve Q _{11 of} the working current follows the setpoint function U with a certain delay, but without being able to take part in its jumps. The change D in the working current better reflects the impact character of the driving forces, as can be seen from the positive spike D ₁ corresponding to the pulse I ₁ and the negative spike D ₂ corresponding to the pulse 1 ₂ . It must be considered surprising that such an impulsive character largely suppresses vibrations of the system, while drive impulses normally give rise to vibration excitation.

While the setpoint function U is shown in FIG. 2 as an analog function, it is also possible to emulate and provide the setpoint function by means of needle pulses. The setpoint function can also be provided by digital coding. In all these cases, the valve arrangement 5 is controlled as indicated by the analog signal U in FIG. 2.

Instead of a simple throttle opening 20, a throttle valve with two throttle edges can also be used in the valve arrangement 5, for example in order to influence both the inflowing and outflowing hydraulic flow. The control of the hydraulic flow supplied to the motor does not have to be done via a constant pressure drop; other types of tax are also applicable. It is essential that at the beginning of a ramp increase - at its initial value Q _1a - a positive, jerky volume flow increase D _i and at the end of the ramp rise - close to the final value Q _1e - a negative jerky volume flow decrease - D _{2 is} superimposed, these volume surges - or a proportion - in relation to the compression volume V _{k of} the system. The sudden increase in volume flow D _{i also} reflects the existence of the pulses I _G and I _R. The pulse I ₁ or the compression volume V _k thus represent only a portion of the total volume of the volume surge D ₁ . The pulse I ₃ and the volume shock D ₃ correspond to each other. Since the slope K ₃ of curve v in FIG. 3 approaches zero with increasing t, 1 ₄ and thus D ₄ also become zero.

The pulse shape shown in Figs. 2 and 3 Control pulses I ₁ , I ₂ , I ₃ is idealized; in reality, the control pulses are ground due to the inductive resistance of the actuating magnet of the proportional throttle of the valve arrangement 5. Added to this is the inertia of the valve spool. This low-pass behavior of the valve arrangement 5 is desirable. If a valve arrangement is used which does not show sufficient low-pass behavior (in this case the actuating range or the actuating speed range of the valve would be violated), then an element with low-pass behavior, e.g. B. an analog or digital low-pass filter 25 connected between the signal generating element 6, 8 and the valve assembly 5. The throttle opening 20 allows a hydraulic flow Q _{11 to} pass, which shows undulations (see FIGS. 2, 3), but no dips (undesired decrease in the hydraulic flow over time during a general current increase as in FIG. 2).

Claims (10)

1. Method for operating a hydrostatic or pneumatic drive system for masses (M) to be accelerated and decelerated, containing a linear or rotary motor (1), an hydraulic or pneumatic pressure source (2, 3) and a valve arrangement (5), the flow opening (20) of which can be controlled to deliver a variable operating flow (Q₁₁) which is subject to varying operating pressures (p) so that a volume of compression (V_k) of the hydraulic or pneumatic medium is produced as a function of the operating pressure to be expected as compared with the initial state, characterised in that the change in the operating flow (Q₁₁) from an initial value (Q_ia) to a final value (Q_1e) takes place as follows:

initially, a sudden increase (D_i) or decrease (-D₃), depending on the sign of the change, in the volume flow takes place at an initial value (Q_1a);

then, in the centre region, the volume flow changes gradually, also in agreement with the sign of the change;

finally, near the final value (Q_1e), a sudden volume decrease (― D₂) or increase, depending on the sign of the change, takes place, the sudden volume flow change (D_i, -D₂, -D₃), or a proportion of the volume flow change, being selected in relation to the volume of compression (V_k).

2. Method according to Claim 1, characterised in that the change in volume flow in the centre region between the initial value (Q_ia) and the final value (Qie) begins with a decrease in the absolute rate of change and changes into an increase in the absolute rate of change.

3. Method according to Claim 1 or 2, characterised in that, with an increase in the operating flow (Q₁₁), particularly during the start-up of a mass (M) at rest, the sudden volume-flow increase (D_i) has a component for taking account of counter pressure (-G) and frictional force (F_R), that is to say is larger overall than corresponds to the volume of compression (V_K).

4. Method according to Claim 1 to 3, characterised in that with a decrease in the operating flow (Q₁₁), particularly with a decrease in a moving mass (M), the sudden volume increase near the final value (Q_1e) is smaller, with regard to the fading counter pressure, than corresponds to the volume of compression (V_K) and there is no sudden volume increase particularly with a deceler- action to zero.

5. Hydrostatic or pneumatic drive system for masses (M) to be accelerated or decelerated, containing a linear or rotary motor (1), an hydraulic or pneumatic pressure source (2, 3), a valve arrangement (5) the flow opening (20) of which can be controlled for delivering a variable operating flow (Q₁₁) which is subject to alternating operating pressures (p) so that a volume of compression (V_K) of the hydraulic or pneumatic medium is produced as a function of the operating pressure to be expected, as compared with the initial state, and a set-value control generator (6) for controlling the valve arrangement (5), characterised in that the setvalue control generator (6) produces a distance- and timedependent ramp signal (U) including a superimposed positive control pulse (I₁) at the beginning of the ramp rise or at the end of the ramp drop and including a superimposed negative control pulse (1₂) at the end of the ramp rise or (1₃) at the beginning of the ramp drop, the magnitudes of the positive and negative control pulses being selected in dependence on the respective volume of compression (V_K).

6. Drive system according to Claim 5, characterised in that, when starting up a mass (M) at rest, further pulses (I_G, I_R) are superimposed on the positive control pulse (I₁) to take account of a counterpressure (G) and of frictional forces (F_R).

7. Drive system according to Claim 5 or 6, characterised in that there is no positive control pulse in the case where masses are to be decelerated to zero.

8. Drive system according to one of Claims 5 to 7, characterised in that the set-value control generator (6) comprises a pulse generator for positive pulses, a pulse generator for negative pulses and a ramp generator, the values of which can be adjusted externally.

9. Drive system according to Claim 8, characterised in that a microprocessor (8) is provided for controlling the pulse generators and the ramp generator, in which arrangement the microprocessor can be supplied with the values concerning the input flow at the beginning and the end of the ramp (Qi_e, Q_ia), the hydraulic capacity (Ci), the mass (M) to be driven, the piston area (Ai.₂) of the motor (1) and the length in time (t_R) or the slope of the ramp (Ks).

10. Drive system according to one of Claims 5 to 9, characterised in that a low-pass filter (25) is connected between the set-value control generator (6) and the valve arrangement (5).

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